DESCRIPTION

The
rcmd() function is used by the superuser to execute a command on a remote machine using an authentication scheme based on privileged port numbers. The
rresvport() function returns a descriptor to a socket with an address in the privileged port space. The
iruserok() and
ruserok() functions are used by servers to authenticate clients requesting service with
rcmd(). All four functions are used by the
rshd(8) server (among others).

rcmd()

The
rcmd() function looks up the host
*ahost using
gethostbyname(3), returning -1 if the host does not exist. Otherwise
*ahost is set to the standard name of the host and a connection is established to a server residing at the well-known Internet port
inport.

If the connection succeeds, a socket in the Internet domain of type SOCK_STREAM is returned to the caller, and given to the remote command as stdin and stdout. If fd2p is nonzero, then an auxiliary channel to a control process will be set up, and a descriptor for it will be placed in *fd2p. The control process will return diagnostic output from the command (unit 2) on this channel, and will also accept bytes on this channel as being UNIX signal numbers, to be forwarded to the process group of the command. If fd2p is 0, then the stderr (unit 2 of the remote command) will be made the same as the stdout and no provision is made for sending arbitrary signals to the remote process, although you may be able to get its attention by using out-of-band data.

The protocol is described in detail in rshd(8).

rresvport()

The
rresvport() function is used to obtain a socket with a privileged port bound to it. This socket is suitable for use by
rcmd() and several other functions. Privileged ports are those in the range 0 to 1023. Only a privileged process (
CAP_NET_BIND_SERVICE) is allowed to bind to a privileged port. In the glibc implementation, this function restricts its search to the ports from 512 to 1023. The
port argument is value-result: the value it supplies to the call is used as the starting point for a circular search of the port range; on (successful) return, it contains the port number that was bound to.

iruserok() and ruserok()

The
iruserok() and
ruserok() functions take a remote host's IP address or name, respectively, two usernames and a flag indicating whether the local user's name is that of the superuser. Then, if the user is
not the superuser, it checks the
/etc/hosts.equiv file. If that lookup is not done, or is unsuccessful, the
.rhosts in the local user's home directory is checked to see if the request for service is allowed.

If this file does not exist, is not a regular file, is owned by anyone other than the user or the superuser, or is writable by anyone other than the owner, the check automatically fails. Zero is returned if the machine name is listed in the hosts.equiv file, or the host and remote username are found in the .rhosts file; otherwise iruserok() and ruserok() return -1. If the local domain (as obtained from gethostname(2)) is the same as the remote domain, only the machine name need be specified.

If the IP address of the remote host is known, iruserok() should be used in preference to ruserok(), as it does not require trusting the DNS server for the remote host's domain.

*_af() variants

All of the functions described above work with IPv4 (
AF_INET) sockets. The "_af" variants take an extra argument that allows the socket address family to be specified. For these functions, the
af argument can be specified as
AF_INET or
AF_INET6. In addition,
rcmd_af() supports the use of
AF_UNSPEC.

RETURN VALUE

The
rcmd() function returns a valid socket descriptor on success. It returns -1 on error and prints a diagnostic message on the standard error.

The rresvport() function returns a valid, bound socket descriptor on success. It returns -1 on error with the global value errno set according to the reason for failure. The error code EAGAIN is overloaded to mean "All network ports in use."

For information on the return from ruserok() and iruserok(), see above.

VERSIONS

The functions
iruserok_af(),
rcmd_af(),
rresvport_af(), and
ruserok_af() functions are provide in glibc since version 2.2.

CONFORMING TO

Not in POSIX.1-2001. Present on the BSDs, Solaris, and many other systems. These functions appeared in 4.2BSD. The "_af" variants are more recent additions, and are not present on as wide a range of systems.

BUGS

iruserok() and
iruserok_af() are declared in glibc headers only since version 2.12.

SEE ALSO

rlogin(1),
rsh(1),
intro(2),
rexec(3),
rexecd(8),
rlogind(8),
rshd(8)

COLOPHON

This page is part of release 3.53 of the Linux
man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.